Catalytic humidifier and heater, primarily for humidification of the oxidant stream for a fuel cell

ABSTRACT

A method and apparatus are provided for humidifying oxidant supplied to a fuel cell system, which can be a single fuel cell or a multiplicity of fuel cells. A catalytic reactor is provided, which is supplied with a portion of the fuel and the oxidant. The oxidant is supplied in excess of the fuel to the reactor, so as to generate a stream of oxidant which is both heated and humidified. For an air breathing stack, this heated oxidant can simply be mixed with air flowing through the fuel cell stack. For a closed system, a heated and humidified oxidant flow, and optionally a fuel flow are mixed with additional flows of these gases supplied to the fuel cell.

FIELD OF THE INVENTION

[0001] This invention relates to electrochemical fuel cells, moreparticularly electrochemical fuel cells which employ hydrogen as a fueland receive an oxidant to convert the hydrogen to electricity and heat.This invention is even more particularly concerned with thehumidification requirements of such an electrochemical fuel cellemploying a proton exchange membrane.

BACKGROUND OF THE INVENTION

[0002] Generally, a fuel cell is a device which converts the energy of achemical reaction into electricity. It differs from a battery in thatthe fuel cell can generate power as long as the fuel and oxidant aresupplied.

[0003] A fuel cell produces an electromotive force by bringing the fueland oxidant into contact with two suitable electrodes and anelectrolyte. A fuel, such as hydrogen gas, for example, is introduced ata first electrode where it reacts electrochemically in the presence ofthe electrolyte and catalyst to produce electrons and cations in thefirst electrode. The electrons are circulated from the first electrodeto a second electrode through an electrical circuit connected betweenthe electrodes. Cations pass through the electrolyte to the secondelectrode. Simultaneously, an oxidant, typically air, oxygen enrichedair or oxygen, is introduced to the second electrode where the oxidantreacts electrochemically in presence of the electrolyte and catalyst,producing anions and consuming the electrons circulated through theelectrical circuit; the cations are consumed at the second electrode.The anions formed at the second electrode or cathode react with thecations to form a reaction product such as water. The first electrode oranode may alternatively be referred to as a fuel or oxidizing electrode,and the second electrode may alternatively be referred to as an oxidantor reducing electrode. The half-cell reactions at the two electrodesare, respectively, as follows:

[0004] First Electrode: H₂→2H⁺+2e⁻

[0005] Second Electrode: {fraction (1/2)}O₂+2H⁺+2e⁻→H₂O

[0006] The external electrical circuit withdraws electrical current andthus receives electrical power from the cell. The overall fuel cellreaction produces electrical energy which is the sum of the separatehalf-cell reactions written above. Water and heat are typicalby-products of the reaction.

[0007] In practice, fuel cells are not operated as single units. Rather,fuel cells are connected in series, stacked one on top of the other, orplaced side by side. A series of fuel cells, referred to as fuel cellstack, is normally enclosed in a housing. The fuel and oxidant aredirected through manifolds to the electrodes, while cooling is providedeither by the reactants or by a cooling medium. Also within the stackare current collectors, cell-to-cell seals and insulation, with requiredpiping and instrumentation provided externally of the fuel cell stack.The stack, housing, and associated hardware make up the fuel cellmodule.

[0008] Fuel cells may be classified by the type of electrolyte, which iseither liquid or solid. The present invention is primarily concernedwith fuel cells using a solid electrolyte, such as a proton exchangemembrane (PEM). The PEM has to be kept moist with water because themembranes that are currently available will not operate efficiently whendry. Consequently, the membrane requires constant humidification duringthe operation of the fuel cell, normally by adding water to the reactantgases, usually hydrogen and air.

[0009] The proton exchange membrane used in a solid polymer fuel cellacts as the electrolyte as well as a barrier for preventing the mixingof the reactant gases. An example of a suitable membrane is acopolymeric perfluorocarbon material containing basic units of afluorinated carbon chain and sulphonic acid groups. There may bevariations in the molecular configurations of this membrane. Excellentperformances are obtained using these membranes if the fuel cells areoperated under fully hydrated, essentially water-saturated conditions.As such, the membrane must be continuously humidified, but at the sametime the membrane must not be over humidified or flooded as thisdegrades performances. Furthermore, the temperature of the fuel cellstack must be kept above freezing in order to prevent freezing of thestack.

[0010] Cooling, humidification and pressurization requirements increasethe cost and complexity of the fuel cell, reducing its commercial appealas an alternative energy supply in many applications. Accordingly,advances in fuel cell research are enabling fuel cells to operatewithout reactant conditioning, and under air-breathing, atmosphericconditions while maintaining usable power output.

[0011] The current state-of-the-art in fuel cells, although increasinglyfocusing on simplified air-breathing, atmospheric designs, has notadequately addressed operations in sub-zero temperatures, which requiresfurther complexity in the design. For instance, heat exchangers andthermal insulation are required, as are additional control protocols forstartup, shut-down, and reactant humidifiers.

[0012] Where a solid polymer proton exchange membrane (PEM) is employed,this is generally disposed between two electrodes formed of porous,electrically conductive material. The electrodes are generallyimpregnated or coated with a hydrophobic polymer such aspolytetrafluoroethylene. A catalyst is provided at eachmembrane/electrode interface, to catalyze the desired electrochemicalreaction, with a finely divided catalyst typically being employed. Themembrane/electrode assembly is mounted between two electricallyconductive plates, each which has at least one (fluid) flow passageformed therein. The fluid flow conductive fuel plates are typicallyformed of graphite. The flow passages direct the fuel and oxidant to therespective electrodes, namely the anode on the fuel side and the cathodeon the oxidant side. The electrodes are electrically connected in anelectric circuit, to provide a path for conducting electrons between theelectrodes. Electrical switching equipment and the like can be providedin the electric circuit as in any conventional electric circuit. Thefuel commonly used for such fuel cells is hydrogen, or hydrogen richreformate from other fuels (“reformate” refers to a fuel derived byreforming a hydrocarbon fuel into a gaseous fuel comprising hydrogen andother gases). The oxidant on the cathode side can be provided from avariety of sources. For some applications, it is desirable to providepure oxygen, in order to make a more compact fuel cell, reduce the sizeof flow passages, etc. However, it is common to provide air as theoxidant, as this is readily available and does not require any separateor bottled gas supply. Moreover, where space limitations are not anissue, e.g. stationary applications and the like, it is convenient toprovide air at atmospheric pressure. In such cases, it is common tosimply provide channels through the stack of fuel cells to allow forflow of air as the oxidant, thereby greatly simplifying the overallstructure of the fuel cell assembly. Rather than having to provide aseparate circuit for oxidant, the fuel cell stack can be arranged simplyto provide a vent, and possibly some fan or the like to enhance airflow.

[0013] Catalytic burners are also known and operate on a principlesimilar to fuel cells, but at an accelerated kinetic rate and increasedtemperature. A fuel, for example hydrogen, is oxidized through directcontact with oxygen or air at a rate induced by the presence of acatalytic bed, for example, ceramic beads containing small amounts ofplatinum on the surface.

[0014] The by-product of the chemical reaction is similar to that of afuel cell, but without any generation of electricity:

[0015]^(1/2)O₂+H₂→H₂O+HEAT

[0016] The higher consumption rate of the reactants and concomitant heatrelease reflects the fact that the reaction occurs through directcontact rather than through a proton/electron transaction. Catalyticburning is flameless, and occurs at a temperature between that of a fuelcell's “cold combustion” and that of an open-flame combustion. Flow ratecan be pulsed or modulated to achieve varying temperature profiles.Hydrogen catalytic burning requires no pilot flame or spark to beinitiated.

[0017] An example of a proposal for a catalytic burner is found in anarticle entitled “Catalytic Combustion of Hydrogen in a DiffusiveBurner” by K. Stephen and B. Dahm at pages 1483-1492 of CatalyticCombustion of Hydrogen in a Diffusive Burner.

SUMMARY OF THE INVENTION

[0018] In accordance with a first aspect of the present invention, thereis provided a tubular reactor, for catalyzing the reaction of hydrogenand a gaseous oxidant, the tubular reactor comprising:

[0019] an elongated housing, a catalyst formed from a material adaptedto promote catalytic combustion of the fuel and the oxidant, beingformed into an elongated body substantially filling the elongate housingand being porous, a first inlet for a gaseous fuel and a second inletfor a gaseous oxidant, both first and second inlets being provided atone end of the elongated housing;

[0020] and an outlet at the other end of the housing, whereby, in use,the catalyst promotes combustion between the fuel and the oxidant togenerate heat and moisture, whereby a heated and humidified gas flowexits through the outlet.

[0021] Preferably, the housing and the body of the catalyst are bothgenerally cylindrical and have length substantially longer than thediameter than the tubular reactor.

[0022] In accordance with a second aspect of the present invention,there is provided a fuel cell system comprising at least one fuel cell,each fuel cell comprising:

[0023] an inlet for a fuel;

[0024] an anode having a catalyst associated therewith for producingcations from the fuel;

[0025] a fuel manifold, connected between the inlet and the anode, fordistributing fuel to the anode;

[0026] an oxidant inlet means for supplying oxidant;

[0027] a cathode having a catalyst associated therewith and connected tothe oxidant inlet means, for producing anions from the oxidant, saidanions reacting with said cations to form water on said cathode;

[0028] an ion exchange membrane deposed between said anode and saidcathode, said membrane facilitating migration of cations from said anodeto said cathode, while isolating the fuel and the oxidant from oneanother;

[0029] the catalytic reactor having a first inlet for fuel and a secondinlet for an oxidant, and an outlet for heated and humidified gas, thecatalytic reactor being mounted to supply the heated and humidified gasto the fuel cell.

[0030] Preferably, the fuel cell system comprises a plurality of fuelcells, forming a fuel cell stack.

[0031] The stack can comprise an air-breathing stack, including aplurality of channels extending through the fuel cell stack forpermitting free flow of ambient air as the oxidant through the fuel cellstack, there being at least one channel for each fuel cell, wherein thecatalytic reactor is mounted below the fuel cell stack. The catalyticconverter is configured to receive air as an oxidant through the secondinlet thereof in excess of the stoichiometric quantity of air requiredfor combustion of fuel within the catalytic reactor, whereby heated andhumidified air is discharged from the outlet of the catalytic reactor.The outlet of the catalytic reactor is mounted below the channels of thefuel cell stack, whereby, heated and moistened air flows upwardlythrough the channels of the fuel cell stack from the catalytic reactor.

[0032] The catalytic reactor can be either generally tubular or it canbe disk-shaped, configured for flow of fuel and oxidant generally alongthe central axis of the reactor.

[0033] A further aspect of the present invention provides a method ofoperating a fuel cell system comprising a plurality of fuel cells, eachfuel cell comprising an inlet for fuel, an anode having a catalystassociated therewith for producing cations from fuel, a fuel manifoldconnected between the inlet and the anode for distributing fuel to theanode, an oxidant inlet means for supplying oxidant, a cathode having acatalyst associated therewith and connected to the oxidant inlet meansfor producing anions from the oxidant, said anions reacting with saidcations to form water on said cathode and an ion exchange membranedisposed between the anode and the cathode, for facilitating migrationof cations from the anode to the cathode, while isolating the fuel andoxidant from one another, the method comprising

[0034] (a) supplying oxidant and fuel to the fuel cell for reaction togenerate electrical power and heat;

[0035] (b) supplying fuel to the catalytic reactor and oxidant to thecatalytic reactor, in an amount greater than the stoichiometric amountrequired for the combustion of the fuel, to ensure complete combustionof the fuel, thereby generating a flow of heated and humidified oxidant;

[0036] (c) supplying the heated and humidified oxidant to the fuel cell,for reaction with the fuel to generate electricity and heat.

[0037] For initial start-up below a preset temperature, the method cancomprise initially supplying fuel and oxidant only to the catalyticreactor to generate a flow of heated and moistened oxidant, and passingthe heated and moistened oxidant through the fuel cell to preheat thefuel cell, and commencing supply of fuel to the fuel cell, once the fuelcell reaches a desired temperature. Then, after start-up and after thefuel cell has reached the desired temperature, a sufficient quantity ofthe oxidant and the fuel are supplied to the reactor, to maintain theoxidant supplied by the catalytic reactor to the fuel cell system at adesired humidity level.

[0038] Yet another aspect of the present invention provides a method ofoperating a fuel cell system comprising a plurality of fuel cells, eachfuel cell comprising an inlet for fuel, an anode having a catalystassociated therewith for producing cations from fuel, a fuel manifoldconnected between the inlet and the anode for distributing fuel to theanode, an oxidant inlet means for supplying oxidant, a cathode having acatalyst associated therewith and connected to the oxidant inlet means,for producing anions from the oxidant, said anions reacting with saidcations to form water on said cathode and an ion exchange membranedisposed between the anode and the cathode, for facilitating migrationof cations from the anode to the cathode while isolating the fuel andthe oxidant from one another, the method comprising:

[0039] (a) supplying oxidant and fuel to the fuel cells for reaction togenerate electrical power and heat;

[0040] (b) supplying fuel to the catalytic reactor and oxidant to thecatalytic reactor, in an amount less than the stoichiometric amountrequired for combustion of fuel, to ensure complete consumption of theoxidant, thereby generating a flow of heated and humidified fuel;

[0041] (c) supplying the heated and humidified fuel to the fuel cell,for reaction with oxidant known to generate electricity and heat.

[0042] This aspect of the method can include:

[0043] (a) providing a second catalytic reactor;

[0044] (b) supplying the second reactor with fuel and oxidant in anamount greater than the stoichiometric amount required for combustion offuel, thereby generating a flow of heated and humidified oxidants;supplying the heated and humidified oxidant to the oxidant inlet meansof the fuel cell, for reaction with a heated and humidified fuel togenerate electricity and heat.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0045] For a better understanding of the present invention and to showmore clearly how it may be carried into effect, reference will now bemade, by way of example, to the accompanying drawings which show apreferred embodiment of the present invention and in which:

[0046]FIG. 1 is a schematic view of the first embodiment of a fuel cellsystem in accordance with the present invention;

[0047]FIG. 2 is a schematic view of a second embodiment of a fuel cellsystem in accordance with the present invention;

[0048]FIG. 3 is a schematic view of a third embodiment of a fuel cellsystem in accordance with the present invention;

[0049]FIGS. 4a, 4 b and 4 c are, respectively, perspective and sideviews of a tubular reactor in accordance with the present invention;

[0050]FIG. 5 is a plan view of part of the fuel cell stack of FIGS. 1,2, and 3;

[0051]FIGS. 6a, 6 b and 6 c are, respectively, planned, side and endviews of a manifold for a catalytic hydrogen burner in accordance withthe present invention; and

[0052]FIGS. 7, 8 and 9 are more detailed views of the embodiments of thefuel cell systems shown in FIGS. 1, 2 and 3 respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0053] Referring first to FIGS. 1 and 7, the first embodiment of theapparatus is indicated generally by the reference 10 and includes anenclosure 12, in the apparatus. In the drawings, this is identified as aHyTef-FC15 enclosure.

[0054] Within the enclosure 12, there is a fuel stack 14 comprising, inknown manner, a plurality of PEM fuel cells, and described in greaterdetail in relation to FIG. 5. For the stack 14, a main fuel supply line16 is provided for hydrogen. The fuel line 16 extends into the enclosure12 and continues as a main supply line 18 including a solenoid-operatedvalve 20 and a secondary fuel line 22 including a respective flowcontrol valve 24 and a solenoid-operated valve 25. As shown, a stackpurge outlet at 26 enables excess hydrogen to be purged from the fuelcell stack 14. A respective solenoid controlled valve 27 controlspurging of the hydrogen gas. Again, as is known, this preventsaccumulation of contaminants and impurities in the hydrogen fuel, withinthe fuel cell stack 14. The purged hydrogen through the purge outlet 26can be recycled for consumption.

[0055] The fuel cell stack 14 in FIG. 1 is an air-breathing stack.Hydrogen fuel flows through the anode side of each individual fuel cellin known manner. Correspondingly, arrows 29 indicate the flow of air asan oxidant through the cathode side of each fuel cell and venting ofthis air flow from the top of the stack as indicated at 43 in FIGS. 1and 2. The stack 14 is open at the top and bottom and provided withsuitable vents, to enable free flow of air through the cathode sides ofthe fuel cells, as indicated by arrows 29.

[0056] In accordance with the present invention, the secondary hydrogensupply line 22 is connected to a catalytic reactor 30, which includes acatalytic reactor bed 32, comprising, for example reticulated aluminum;this material is chosen for its thermal conductivity, low cost and easeof use. A separate air inlet 34 is provided, connected via pump 36 andan air supply line 38 to the catalytic burner 30.

[0057] Excess air is supplied by the air pump 36 to the catalyticreactor 30. Hydrogen supplied to the catalytic reactor 30 reacts withthe air, to generate heat. As a consequence, as indicated at 29, thereis discharge from the catalytic reactor 30 of air, water and heat. Inother words, the air supply from inlet 34 has, effectively, been heatedand humidified. This heated humidified air then flows through the stack14.

[0058] The catalytic reactor or burner 30 of FIG. 1 is based, to acertain extent, on conventional configuration. As such, the actualcatalytic reactor bed 32, of known manner, is relatively broad and has alarge surface area, but at the same time it is relatively shallow, interms of the flow direction of the gases. The intention is to relate alarge area for gas flow, which in turn reduces gas velocity. This shouldensure sufficient residence time for substantially complete combustion,as effected by the catalytic reactor bed 32.

[0059] In practice, it is often difficult to achieve completecombustion, and this is important in this application. Moreover, whilethe large surface area of the reactor bed 32 is suitable for an openconfiguration, it is unsuited for any closed system. Instead, theinventors have developed an elongated, tubular reactor, which isindicated at 50 in FIG. 2. This tubular reactor 50 is described ingreater detail below in relation to FIG. 4. The other components in FIG.2 correspond to those in FIG. 1. For simplicity and brevity, likecomponents in FIG. 2 are given the same reference numeral and thedescription of these components is not repeated.

[0060] Thus, the embodiment of FIG. 2, also shown in FIG. 8, functionsin much the same way as the embodiment of FIG. 1. However, instead ofhaving the catalytic reactor 30 with an open surface reactor bed 32 witha large surface area, there is instead provided the enclosed tubularreactor 50. The reactor 50 has respective inlets 52 and 54 for fuel andair, and a tubular outlet 56. A flow of heated, humidified air 40 exitsfrom the tubular outlet 56, and will then flow through the open fuelcell stack 14 as indicated, again, by arrows 29. An air inlet 41 isconnected to an air line 42, including air blower or pump 40.

[0061] A non-return valve 58 and a flash arrester 59 are provided, asfor the next embodiment. Reference will now be made to FIGS. 3 and 7,which show a third embodiment of the present invention. This embodimentof the invention again can have an enclosure as indicated generally bythe reference 60, and again includes a fuel cell stack 62. The stack 62here is a closed stack, and is provided with an air pump or blower 64connected by a main supply line 66 to an inlet of the fuel cell stack62, and excess air exhausts from the fuel cell stack 62 as indicated at68.

[0062] On the hydrogen side, a hydrogen supply line 70 can include apressure gauge and a flow meter (not shown), and comprises a mainhydrogen supply line 72 to the fuel cell stack 62 and a secondary supplyline 74 to the catalytic burner or reactor 50. A solenoid valve 73 isprovided in the main supply line 72, and a solenoid valve 75, a flasharrestor 76 and a non-return valve 77 are provided in the secondary line74. A fuel purge valve 78 with a controlling solenoid valve 79 areprovided as for the first embodiment.

[0063] The tubular reactor 50 is again provided and the hydrogen inlet52 is again provided at the side of the reactor.

[0064] An air supply line for the reactor 50 is indicated at 80 andincludes a pump or meter 82, and a respective non-return valve 84. Theair supply line 80 is connected to a respective inlet 54 on thecatalytic reactor 50. Optionally, a pressure gauge and a flow meter canbe provided (not shown).

[0065] The outlet 56 of the tubular reactor 50 is connected by a line85, to two branch lines 86 and 87, which are connected by respectivesolenoid valves 88 and 89 to the fuel supply line 72 and to the airsupply line 66. Although not shown, the stack 62 can optionally includea recirculation pump. Excess hydrogen can, in a known manner, be purgedthrough the outlet or purge line 78, to prevent build-up ofcontaminants.

[0066] The tubular reactor 50 can be run to provide either a humidifiedand heated flow of air or a humidified and heated flow of hydrogen.These two modes of operation are detailed below.

[0067] To generate a flow of heated and humidified air, excess air isdelivered by the pump 82, relative to the hydrogen flow through the line74. In the tubular reactor 50, the oxygen reacts with the hydrogen togenerate heat and moisture. This results in a heated and moistened airflow exiting through the outlet 56. Then, the valve 88 is maintainedclosed and the valve 89 is opened, so that the heated and moistened airflow passes through to the main air supply line 66, to be entrained intothe air flow passing to the fuel cell stack 62.

[0068] Correspondingly, to generate a heated hydrogen flow, the valve 88is opened and the valve 89 closed. Then, excess hydrogen is suppliedthrough the line 74, as compared to air supplied through the main fuelline 72. The flow is dead ended and is only exhausted during purgingwhen the exhaust solenoid 79 is open. However, the flow can becontrolled using control valves when not operated in dead-ended mode. Inthe tubular reactor 50, the oxygen in the air reacts with some of thehydrogen to generate heat and moisture. This leaves the flow ofhydrogen, with residual nitrogen, together with heat and moisture, tothen exit from the outlet 56. This flow of heated and humidifiednitrogen and hydrogen gas passes through valve 88 into the main fuelline 72.

[0069] It will be appreciated that where heated and humidified hydrogenis supplied to the fuel line 72, and as air is used as the oxidant, thisdoes result in nitrogen being injected into the fuel gas supply. Forthis reason, the purge line 78 will need to be used, to prevent thebuild-up of nitrogen within the fuel cell stack 62. Alternatively aflowing system can be used at all times.

[0070] It is important that, in the reactor 50, complete reaction takesplace. In other words, it is essential that, in the two modes ofoperation, residual hydrogen is not delivered to the main air line 66,nor residual oxygen delivered to the hydrogen supply line 72. This couldresult in potentially flammable gas mixtures of hydrogen and oxygenbeing delivered to the fuel cell stack 60, which is dangerous. To ensurecomplete reaction, proper topology and morphology of the reactor must bedesigned, essentially to ensure adequate residency time over the fullrange of flow rates.

[0071] It will also be understood that it is possible to heat andhumidify both of the fuel and air supply lines. Because of the differentrequirements of the two supply lines, this would require the provisionof two separate tubular reactors, each of which would be configured tooperate in one of the two modes outlined above.

[0072] Turning to FIG. 4, this shows, in detail, the tubular reactor 50.It is to be appreciated that this is an early version of the tubularreactor 50, and in particular, the housing of the reactor 50 is madefrom conventional, off-the-shelf components. It is anticipated that theoverall configuration of the tubular reactor 50 can be enhanced to givea design which both has better performance characteristics, and is moreeconomical to manufacture.

[0073] Referring to FIG. 4, the tubular reactor 50 comprises a tubularreactor housing 51. At the lower end thereof, a T-connector 100 isprovided. The T-connector 100 has three coupling flanges 102, one ofwhich is connected to the tubular housing 51, and the two others ofwhich provide connections for the hydrogen and air supply lines. At thetop end, the tubular reactor 50 includes a connector 104, again providedwith connection flanges 106, one of which is connected to the tubularbody 51 and the other of which provides connection to a supply line.While a housing 51 of circular cross-section is shown, it will beunderstood that any suitable cross-section, for example a squarecross-section, could be used.

[0074] Reference will now be made to FIG. 5. This shows a plan view offive pairs of flow field plates making up five individual fuel cellelements in the fuel cell stack 62. Thus, there are oxidant flow fieldplates indicated at 110. Fuel flow field plates are indicated at 112.Between each pair of oxidant and fuel flow field plates 110,112, thereis located a respective membrane electrode assembly (MEA) and gasdiffusion media 114. Between the oxidant flow field plates 110 and theMEA 114, there are defined oxidant channels 116, and fuel flow hydrogenchannels 118 are defined between the fuel flow field plates 112 and theMEA 114. Cooling channels 120 are provided in the back of the oxidantflow field plates 110, against the fuel flow field plates 112. Thesecooling channels 120 are, like the oxidant channels 116, simply channelsextending vertically (not necessarily vertical) through the stack 62, toprovide free flow of ambient air through the channels. In known manner,other constructional details of the stack, e.g. elements holding thevarious flow field plates together, are not shown, but these can beconventional.

1. A tubular reactor, for catalyzing the reaction of hydrogen and agaseous oxidant, the tubular reactor comprising: an elongated housing, acatalyst formed from a material adapted to promote catalytic combustionof the fuel and the oxidant, being formed into an elongated bodysubstantially filling the elongated housing and being porous, a firstinlet for a gaseous fuel and a second inlet for a gaseous oxidant, bothfirst and second inlets being provided at one end of the elongatedhousing; and an outlet at the other end of the housing, whereby, in use,the catalyst promotes combustion between the fuel and the oxidant togenerate heat and moisture, whereby a heated and humidified gas flowexits through the outlet.
 2. A tubular reactor as claimed in claim 1,wherein the housing and the body of the catalyst are both generallycylindrical and have a length substantially longer than the diameterthereof.
 3. A tubular reactor as claimed in claim 1 or 2, whichincludes, for the first and second inlets, fittings for connection tosupply lines for fuel and the oxidant, and for the outlet, a fitting forconnection to a conduit for receiving the heated, humidified gas flow.4. A fuel cell system comprising: at least one fuel cell, each fuel cellcomprising: an inlet for a fuel; an anode having a catalyst associatedtherewith for producing cations from the fuel; a fuel manifold,connected between the inlet and the anode, for distributing fuel to theanode; an oxidant inlet means for supplying oxidant; a cathode having acatalyst associated therewith and connected to the oxidant inlet means,for producing anions from the oxidant, said anions reacting with saidcations to form water on said cathode; an ion exchange membrane deposedbetween said anode and said cathode, said membrane facilitatingmigration of cations from said anode to said cathode, while isolatingthe fuel and the oxidant from one another; and a catalytic reactorhaving a first inlet for fuel and a second inlet for an oxidant, and anoutlet for heated and humidified oxidant, the catalytic reactor beingmounted to supply the heated and humidified oxidant to the fuel cell. 5.A fuel cell system as claimed in claim 4, which includes a plurality offuel cells forming a fuel cell stack, which includes a main fuel inletconnected to all of the inlets of the fuel cells.
 6. A fuel cell systemas claimed in claim 5, wherein the fuel cell stack is an air-breathingstack, including a plurality of channels extending upwardly through thefuel cell stack for permitting free flow of ambient air as the oxidantthrough the fuel cell stack, there being at least one channel for eachfuel cell and the oxidant inlet means being provided by the lower endsof the channels, wherein the catalytic reactor is mounted below the fuelcell stack and is configured to receive air as an oxidant through thesecond inlet thereof in excess of the stoichiometric quantity of airrequired for combustion of fuel within the catalytic reactor, wherebyheated and humidified air is discharged from the outlet of the catalyticreactor, and wherein the outlet of the catalytic reactor is mountedbelow the channels of the fuel cell stack, whereby, heated and moistenedair flows upwardly through the channels of the fuel cell stack from thecatalytic reactor.
 7. A fuel cell system as claimed in claim 6 whichincludes a supply line for fuel connected to both the main fuel inlet ofthe fuel cell stack and the first inlet of the catalytic reactor, andwhich includes an air supply line including an air delivery deviceconnected to the second inlet of the catalytic reactor.
 8. A fuel cellsystem as claimed in claim 6 or 7 wherein the catalytic reactor isgenerally tubular.
 9. A fuel cell system as claimed in claim 6 or 7,wherein the catalytic reactor includes a generally disc shaped reactorelement, configured for flow of fuel and oxidant, generally along acentral axis thereof.
 10. A fuel cell system as claimed in claim 5wherein the outlet of the catalytic reactor is connected by a firstcontrol valve to the main fuel inlet of the fuel cell stack and by asecond control valve to the oxidant inlet means whereby, in use, theoutlet of the catalytic reactor can be selectively connected to one ofthe main fuel inlet and the oxidant inlet means, with supply of theoxidant as the fuel to the catalytic reactor adjusted so that the heatedand humidified gas at the outlet of the catalytic reactor includes anexcess of gas corresponding to said one of the main fuel inlet and theoxidant inlet means.
 11. A fuel cell system as claimed in claim 10,wherein each of the fuel supply line and the air supply line include atleast one of a pressure gauge, a flow meter and a non-return valve. 12.A fuel cell system as claimed in claim 5, wherein the oxidant inletmeans comprises an air distribution manifold within the fuel cell stackfor distributing air, as the oxidant, to individual fuel cells, whereina main air supply line is provided connected to the air distributionmanifold, and a main fuel line is connected to the fuel inlet, andwherein a secondary fuel line branches off from the main fuel supplyline and is connected to the catalytic reactor for supplying fuel.
 13. Afuel cell system as claimed in claim 12, wherein the fuel cell stackincludes a fuel outlet and means for recirculating fuel from the fueloutlet to the fuel inlet.
 14. A fuel cell system as claimed in claim 12,wherein the catalytic reactor is provided in the main air supply line,and wherein a second catalytic reactor is provided in the fuel cell lineand a secondary air supply line connects the main air supply line to thesecondary catalytic reactor, for a supply of air in an amount less thanthe stoichiometric amount required for combustion of fuel, whereby, thesecondary catalytic reactor generates heated and humidified fuel.
 15. Afuel cell system as claimed in claim 14, where each of the first andsecond catalytic reactors is generally tubular.
 16. A method ofoperating a fuel cell system comprising a plurality of fuel cells, eachfuel cell comprising an inlet for fuel, an anode having a catalystassociated therewith for producing cations from fuel, a fuel manifoldconnected between the inlet and the anode for distributing fuel to theanode, an oxidant inlet means for supplying oxidant, a cathode having acatalyst associated therewith and connected to the oxidant inlet meansfor producing anions from the oxidant, said anions reacting with saidcations to form water on said cathode and an ion exchange membranedisposed between the anode and the cathode, for facilitating migrationof cations from the anode to the cathode while isolating the fuel andoxidant from one another, the method comprising (a) supplying fuel tothe fuel cell for reaction to generate electrical power and heat; (b)providing a catalytic reactor, supplying fuel to the catalytic reactorand supplying oxidant to the catalytic reactor, in an amount greaterthan the stoichiometric amount required for the combustion of the fuel,to ensure complete combustion of the fuel, thereby generating a flow ofheated and humidified oxidant; (c) supplying the heated and humidifiedoxidant to the fuel cell, for reaction with the fuel to generateelectricity and heat.
 17. A method as claimed in claim 16, whichcomprises, for initial start-up below a preset temperature, initiallysupplying fuel and oxidant only to the catalytic reactor to generate aflow of heated and humidified oxidant, and passing the heated andhumidified oxidant through the fuel cell to preheat the fuel cell, andcommencing supply of fuel to the fuel cell, once the fuel cell reaches adesired temperature.
 18. A method as claimed in claim 16, which includesproviding the catalytic reactor in a main oxidant supply line forsupplying oxidant to the fuel cell stack.
 19. A method as claimed inclaim 17, which includes, after start-up and after the cell has reachedthe desired temperature, supplying a sufficient quantity of the oxidantand the fuel to the catalytic reactor, to maintain the oxidant suppliedto the fuel cell system at a desired humidity level.
 20. A method asclaimed in claim 19, which includes supplying air as the oxidant; wherethe fuel cell system is an air-breathing system including verticalchannels for flow of air as the oxidant; and providing only a portion ofair required as the oxidant through the catalytic reactor, withadditional air flowing directly through the channels of the fuel cellsystem.
 21. A method as claimed in claim 16, which includes: (a)providing a second catalytic reactor; (b) supplying the second reactorwith fuel and oxidant in an amount less than the stoichiometric amountrequired for combustion of fuel, thereby generating a flow of heated andhumidified fuel; (c) supplying the heated and humidified fuel to theinlet for fuel of the fuel cell.
 22. A method as claimed in any one ofclaims 16 to 21, which includes providing the fuel cell system withproton exchange membranes.